I wonder if it's because people already associate them with being the cheapest, they want people to just default to going to their site and be too lazy to check

I wonder if it's because people already associate them with being the cheapest, they want people to just default to going to their site and be too lazy to check the comparison sites. If they made it easy to compare, they would lose out on that crowd for any time they weren't the cheapest. Maybe it also allows competitors to beat their prices algorithmically.